Labral tears don't have to be from a single acute incident. They can arise from repetitive strain and wear, particularly if you've been lifting very heavy weights. The downward drag on the humeral head can cause a small lesion on the labrum quite easily.

The SC joint is a very difficult joint to manage, and there is a similar thread in the Sports Physiotherapy section about the SC joint. Found here. .

Wikipedia reference-linkMRI would be great, for excluding labral tear, but it is unlikely to show you an unstable SC joint (unless it shows ligament trauma) your best bet to exclude SC joint is to get it injected with a corticosteroid under radiographic imaging and then see if the pain disappears. It will not stop the clunking, but it will tell you 100% if the SC is the cause of the pain.

Advice: Find a good sports physio. Tell them your suspicions. Get them to assess, and see if you can get an MRI, and if the physio thinks it's indicated a cortico shot. If the cortico works organise a referral to agood orthopod who specialises in shoulders.